Superalloys and heat resistant alloys are materials that exhibit superior mechanical and environmental attack resistance properties at elevated temperatures. Typically, they include as their main constituents: nickel, chromium, cobalt and iron either singly or in combinations thereof. Other materials are added to the alloys to impart additional desired characteristics.
The properties of such alloys are strongly affected by their grain size. At relatively low temperatures, smaller grain sizes are generally acceptable. However, at elevated temperatures (about 1600.degree. F. or 870.degree. C. and higher) creep is usually observed to occur much more rapidly in fine grain materials than in coarse grain materials. Accordingly coarse grain materials are usually preferred for high temperature applications. For example, turbine blades are exposed to hellish environments (about 1800.degree. F. or 980.2.degree. C. or higher) and, as a consequence, require coarse, elongated grain structures.
One method used for improving the properties of an alloy is to form elongated grains. By encouraging grain elongation there are relatively fewer grain boundaries transverse to the stress axis. Elongated grain boundaries appear to improve both the creep and high temperature properties of the alloy.
Oxide dispersion strengthened ("ODS") alloys made by mechanical alloying techniques exhibit superior high temperture rupture strength due to the presence of stable oxide particles in a coarse and highly elongated grain matrix.
A common method for achieving directional recrystallization is called zone annealing. See U.S. Pat. No. 3,746,581 (Cairns, et al). Briefly, zone annealing is routinely applied to constant cross section barstock in order to promote the development of the requisite coarse, elongated grain structure needed for high temperature strength. However, with respect to forgings, which are generally short and irregular, temperature control is difficult. Moreover, thermal gradients in the forgings, an essential feature of zone annealing, are variable and are generally lower than optimum values. It is often a difficult and expensive undertaking to either propel the forging through a distinct temperature zone in a furnace or, conversely, direct a travelling temperature zone across the forging.